Surface modification of PET film by plasma-based ion implantation

Surface modification of PET film by plasma-based ion implantation

Nuclear Instruments and Methods in Physics Research B 206 (2003) 687–690 www.elsevier.com/locate/nimb Surface modification of PET film by plasma-based ...

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Nuclear Instruments and Methods in Physics Research B 206 (2003) 687–690 www.elsevier.com/locate/nimb

Surface modification of PET film by plasma-based ion implantation N. Sakudo

a,*

, D. Mizutani a, Y. Ohmura a, H. Endo a, R. Yoneda a, N. Ikenaga b, H. Takikawa c

a

c

Kanazawa Institute of Technology, Advanced Materials Science R & D Center, 3-1 Yatsukaho, Matto, Ishikawa 924-0838, Japan b Shibuya Kogyo Co. Ltd., 2-232 Wakamiya, Kanazawa, Ishikawa 920-0054, Japan Toyohashi University of Technology, Department of Electrical & Electronic Engineering, Toyohashi, Aichi 441-8580, Japan

Abstract It has been reported that thin diamond like carbon (DLC) coating is very effective for enhancing the barrier characteristics of polyethylene terephthalate (PET) against CO2 and O2 gases. However, coating technique has a problem of DLC-deposit peeling. In this research, we develop a new technique to change the PET surface into DLC by ion implantation instead of coating the surface with the DLC deposit. The surface of PET film is modified by plasma-based ion implantation using pulse voltages of 10 kV in height and 5 ls in width. Attenuated total reflection FT-IR spectroscopy shows that the specific absorption peaks for PET decrease with dose, that is, the molecules of ethylene terephthalate are destroyed by ion bombardment. Then, laser Raman spectroscopy shows that thin DLC layer is formed in the PET surface area. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 05.57.Ty; 33.20.Fb; 68.47.Mn; 68.60.)p Keywords: PET; Ion implantation; FT-IR; Raman; DLC; Plasma

1. Introduction Polyethylene terephthalate (PET) is widely used as beverage containers and food or medicine packages due to the easy handling as well as the low cost. It is also suitable for recycling. However, the barrier characteristics against a certain kind of gases such as CO2 and O2 are not so good that the * Corresponding author. Tel.: +81-76-274-9262; fax: +81-76274-9251. E-mail address: [email protected] (N. Sakudo).

long-period maintenance of quality is hard for some beverages, beers, wines and medicines. The use of other polymer, polyethylene naphthalate (PEN) is effective to some extent for resolving the gas barrier problem [1]. But, the cost is higher and the material is not suitable for recycling. Recently, an European company developed a new technique that deposits thin diamond like carbon (DLC) on the inner surface of a PET bottle in order to enhance the gas barrier characteristics [2]. It has shown that thin-DLC deposit of 0.02–0.04 lm enhances the barrier characteristics more than ten times as large as the uncoated PET. However, as

0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00824-3

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far as they use coating technique, there remains a peeling problem of the deposit [3]. In this research we propose to use surface modification by ion implantation in order to simultaneously resolve the both problems, the low gas barrier and the DLC-deposit peeling. The possibility of realizing this technique is experimentally investigated.

2. Experimental setup Ion implantation into PET film is carried out with the experimental equipment as shown in Fig. 1. Nitrogen plasma is generated at a pressure of around 102 Pa by off-resonant microwave discharge in magnetic field [4]. Plasma density of 1010 –1011 cm3 is obtained with microwave power of 50–200 W. Usually, microwave nitrogen plasma generated at this condition consists of roughly equal amounts of Nþ and Nþ 2 ions [5]. The ratio of atomic ion Nþ increases with higher microwave power and/or at lower pressure. Sometimes, ion implantation into insulator has a problem due to ion-charge accumulation on the surface. This often causes big troubles in the ion implantation processes of mass-production factories for semiconductor devices [6,7]. However, plasma-based ion implantation that performs the implantation in pulses resolves this problem. Ion charges accumulate on the sample surface during the on time of the high-voltage pulse. But during

the off time the charges are quickly neutralized by the electrons from plasma. A piece of PET film, 10 mm square in area and 0.1 mm in thickness, is placed on the sample holder in the plasma. The sample holder, which is made of copper, is supplied with negative high-voltage pulses of 10 kV in height and 5 ls in width. Since the pulse repetition is 1000 Hz, the duty of the high voltage is 0.5%. Just at the instant when the high voltage is supplied to the sample holder, the PET surface facing the plasma is elevated to the same potential as the sample holder, since the capacitance between the back and the front of PET film is much larger than that between the PET front surface and the vacuum-chamber inner surface (earthed). But then, the front-surface potential against the sample holder, Vc , which is determined with the surface charge and the PET film capacitance, increases as positive ions are implanted into PET. The potential-increase rate is proportional to the plasma density. From a simple calculation, it is easily found that in case of the plasma of 1011 cm3 in density and 10 eV in electron temperature, the PET surface potential against the holder becomes about 600 V at the end of high-voltage pulse. Accordingly, the implant energy varies from 10 to 9.4 keV during 5 ls of the pulse duration. This value of energy variation does not cause any significant problem to the surface modification of PET as far as concerned with our purpose described above.

3. Characterization of ion-implanted PET films

Fig. 1. Experimental equipment for plasma-based ion implantation. Plasma is generated by microwave discharge in magnetic field.

Ion implanted PET films are measured by FTIR spectrometer in order to see how the organic structure of PET is destroyed by ion irradiation. Since the modified layer is very thin, we use attenuated total-reflection type (ATR) FT-IR spectrometer (Perkin Elmer, 1650PC-DC). The prism (or internal reflection element) material is KRS-5. Infrared absorption spectra of surface-modified PET films are shown in Fig. 2. In this figure, the peaks at the wave numbers, 1714, 1250, 1120, 1046 and 726 cm1 are specific for PET. The absorption spectrum obtained by ATR method is generally considered lacking quantitativity, since the signals are very changeable depending on the contact

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of Figs. 2 and 3 we can see that a considerable proportion of PET molecules in the modified layer are destroyed and changed into DLC as discussed in the following section.

4. Discussions

Fig. 2. Infrared absorption spectra of ion implanted PET films. Specific absorption peaks for PET are reduced with dose.

conditions between the prism and the sample surface. Thus, in order to get a relative comparison of the absorption spectra we have to mount each sample on the instrument under the same contact conditions as possible. From Fig. 2 we can see the tendency that the specific absorption peaks decrease with dose. Then, the same samples are measured by a laser Raman spectrometer (Jusco, NRS-1000). The wave length is 532 nm. Fig. 3 shows the Raman spectrum of the ion implanted PET film with the dose of 1.1  1016 cm2 . The spectrum has an asymmetric broad peak around 1550 cm1 . Such a spectrum with an asymmetric broad peak is often seen on the ion plated carbon deposit that is called amorphous carbon or DLC [8]. From the spectra

Regarding the measurement by ATR FT-IR spectroscopy as shown in Fig. 2, the specific peaks for PET decrease with dose. Up to the dose of 6.4  1016 cm2 , there is no indication that the specific peaks would become zero with further higher doses. However, this does not mean that a lot of complete organic structures remain in the ion-implanted layer. Because the modified layer thickness is much thinner than the penetration depth of the evanescent wave used for the measurement. The mean projected range of 10 keV Nþ 2 ions that are implanted into PET is calculated with the simulation software TRIM-98 to be about 0.04 lm, assuming the PET density of 1.33 g cm-3 . As mentioned above, implanted ions contain a lot of Nþ ions with same energy. Therefore, the modified-layer thickness will be about 0.1 lm, if we take the mean projected range of Nþ ions and their standard deviation into account. On the other hand, the penetration depths of the evanescent waves differ with their wave numbers, that is, the depth is inversely proportional to the wave number. Typically, the penetration depth for the wave number of 1000 cm1 is 0.6 lm, which is about six times as large as the ion-implanted layer thickness. Thus, the absorption spectra in Fig. 2 are considered to be the summation of those from the ion implanted layer and from the un-implanted part beneath the implanted layer. The power density P of the evanescent wave inside PET is expressed as a function of the depth z as follows: P ¼ P0  expðz=dp Þ;

Fig. 3. Raman shift of ion implanted PET sheet with the dose of 1.1  1016 cm2 .

ð1Þ

where P0 is the wave power density on the PET surface, and dp , the penetration depth of the wave power. Since the wave power density is concentrated near the surface area, the ratio of spectrum signals from the two parts is determined not by the depths ratio but by the ratio of the depth integrations of Eq. (1). Therefore, as far as we observe

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only the ion implanted layer, a considerable proportion of PET molecules is destroyed. The result by a laser Raman spectroscopy as shown in Fig. 3 confirms this speculation. The Raman spectrum is very similar to those of typical DLCs, which are made by plasma PVD or ion plating using hydrocarbon gases such as methane, ethane or ethylene. Since the wave length in the laser Raman spectrometer (532 nm) is roughly one order shorter than that in the FT-IR, the penetration depth is considered sufficiently smaller than the thickness of ion implanted layer.

characteristics, we consider that they will be at least comparable to those tested with plasmacoating technique by an European company [1], because the thickness of the ion-implanted PET layer by the new technique is almost the same or slightly larger and the Raman structure is very close to those of the typical DLCs. Practical method to apply PBII to modifying the inner surface of a PET bottle is mentioned elsewhere [9].

References 5. Conclusions In order to enhance the barrier characteristics of PET film against CO2 and O2 gases, a new technique for surface modification was investigated. Plasma-based ion implantation (PBII) of nitrogen plasma was proved to be able to change PET-film surface into DLC that is similar to those made by plasma PVD or ion plating. Since the technique is basically the materials modification by ion implantation differently from coating, there will be no peeling problems. Regarding the gas barrier

[1] H. Forcinio, Prepared Foods (March) (1997) 75. [2] J. Briem, Verpackungs-Runds. (6) (1999) 42. [3] B. Olliver, S.J. Dowey, S.J. Young, A. Matthews, J. Adhesion, Sci. Technol. 9 (6) (1995) 769. [4] N. Sakudo, Rev. Sci. Instr. 71 (2000) 1016. [5] N. Sakudo, K. Tokiguchi, T. Seki, Mater. Sci. Eng. A 116 (1989) 221. [6] C.M. McKenna, Radiat. Eff. 44 (1979) 93. [7] G. Ryding, M. Farley, Nucl. Instr. and Meth. 189 (1981) 295. [8] M. Iwaki, M. Matsunaga, K. Terashima, H. Watanabe, in: Ion Implantation Technology-98, 1999, p. 1026. [9] N. Sakudo, N. Ikenaga, Kanazawa Institute of Technology, Japanese Patent Application No. 2000-156572.